1. Field of the Invention
This invention relates to analog to digital (A/D) converters and more particularly to RSD A/D converters.
2. Related Art
RSD (redundant signed digit) A/D converters have been found to be effective for situations in which high speed is useful.
Referring to FIG. 1, a block diagram of a cyclic RSD A/D converter 10, such as the one disclosed in U.S. Pat. No. 5,644,313, is shown. The A/D converter 10 includes an analog section having two RSD stages 11 and 12 followed by a digital section 14 having an alignment and synchronization block 15 and a correction block 16. An analog input signal (voltage) is input to the first RSD stage 11 by way of a switch 18. After the input signal is received, the switch 18 is opened. The first RSD stage 11 compares the input signal to a high voltage (VH) and a low voltage (VL) and generates a first digital output signal, in this case the msb, based on the comparison results. The first RSD stage 11 also generates a first residue voltage VR1. The msb is output to the digital section 14 and the residue voltage VR 1 is input to the second RSD stage 12. The second RSD stage 12 also performs high and low voltage comparison operations, generates a second digital output signal (msb-1), and a second residue voltage VR2. The second digital output signal (msb-1) is output to the digital section 14, the switch 18 is moved to connect the feedback path, and the second residue voltage VR2 is provided to the first RSD stage 11. This operation is repeated, with the RSD stages 11, 12 outputting additional digital bits of the input signal. The digital bits are aligned, synchronized, and combined in the digital section 14 to provide a standard format binary output code.
In some applications, although speed is desired, it is also desirable for there to be high accuracy of the A/D converter. One of the difficulties in providing high precision in a RSD A/D converter is there required a high performance operational amplifier. The operational amplifier should have extremely gain and no offset voltage between the plus and minus inputs thereof. This is very difficult to achieve in practice. The offset voltage is a problem that results in reduced accuracy in the output. This error may be small but may become significant depending upon the application. It can be a limitation in the resolution of the A/D converter. Thus, there is a need for RSD converters with improved resolution.